The present invention relates to nuclear magnetic resonance spectroscopy and imaging.
Upon interaction of a radio-frequency electromagnetic pulse with an atomic nucleus in a specimen in a polarizing magnetic field, a characteristic, so-called free-induction decay (FID) resonance signal is produced which can be used for spectroscopic chemical analysis. Such analysis derives mainly from a phenomenon known as chemical shift which is defined as the relative difference between the strength of an external magnetic field and a resulting field at a nucleus; chemical shift is understood as caused by a shielding or impeding influence of the structure of electrons (and nuclei) surrounding a nucleus in an atom (or molecule). As shielding influence may characterize a chemical species, chemical shift may be interpreted in terms of the presence of chemical elements and compounds in a specimen. Chemical analysis may further derive from other interactions, e.g., spin-spin coupling, either "through-bond" (known as "J-coupling") or "through-space" (known as "dipolar coupling"). Under suitable conditions, chemical analysis can be made spatially selective, allowing for 1-, 2-, or 3-dimensional imaging of the local concentration of a nuclear species--hydrogen, for example, or phosphorus.
Often, in nuclear-magnetic-resonance spectroscopy (NMR) or imaging (MRI), a signal of interest may be difficult to acquire due to a dominant response from an extraneous species which may mask or hide the sought-for signal. This is true even in those cases where the dominant signal does not overlap, partially or totally, with the sought signal. The need for accommodating, through the dynamic-range-limited aperture of the detector, the overall dynamic range of a signal dominated by a solvent peak may necessitate a lower gain. This, in turn, may render the smaller signals too small to detect, i.e., below the noise level of the detector. For example, in the case of a specimen in hydrous solution, a dominant response in the form of a "solvent peak" may originate with hydrogen nuclei in water. Thus, in signal acquisition, there is interest in extracting or isolating a signal of interest from a response which may further include one or several extraneous components.
The following disclosures are cited for addressing this and other aspects of nuclear magnetic resonance technology:
U.S. Pat. No. 4,081,742, issued Mar. 28, 1978 to D. C. Hofer et al., disclosing use of a sequence of three radio-frequency signals to reduce a solvent peak in cases where relaxation time of a dominant solvent species is significantly greater than relaxation time of a species of interest;
U.S. Pat. No. 4,319,190, issued Mar. 9, 1982 to T. R. Brown, disclosing simultaneous imaging of several nuclear species;
U.S. Pat. No. 4,629,988, issued Dec. 16, 1986 to P. A. Bottomley, disclosing localized, sub-surface NMR analysis, including application of a "chemical suppression pulse" for selective irradiation of an undesired intense resonance in the chemical-shift spectrum;
U.S. Pat. No. 4,639,671, issued Jan. 27, 1987 to A. Macovski, disclosing simultaneous acquisition of NMR information from a plurality of points in a region of interest, involving use of a time-varying magnetic gradient field;
U.S. Pat. No. 4,665,366, issued May 12, 1987 to A. Macovski, disclosing a further method for the simultaneous acquisition of NMR information from a plurality of points in a region of interest, involving a sequence of radio-frequency excitations in the presence of different time-varying magnetic gradient fields;
U.S. Pat. No. 4,678,995, issued Jul. 7, 1987 to M. J. Avison, disclosing suitable first, second, and third pulse trains as applied to a sample, and differencing of responses obtained with and without the third signals to eliminate a signal component due to water;
U.S. Pat. No. 4,680,546, issued Jul. 14, I987 to C. L. Dumoulin, disclosing a method for suppressing an NMR signal component due to water, based on a distinction between species as to "quantum coherence"--water being characterized as having single quantum coherence, in contrast to species having zero or multiple quantum coherence;
U.S. Pat. No. 4,728,889, issued Mar. 1, 1988 to D. G. Gadian et al., disclosing a two-pulse-train method for distinguishing a species having two coupled resonances from second and third species, each of which has a resonance which coincides with one of the coupled resonances;
U.S. Pat. No. 4,771,242, issued Sep. 13, 1988 to D. A. Lampman, disclosing a method for selectively stimulating nuclei of interest, without stimulating solvent nuclei, involving judicious selection of magnetic field strength;
U.S. Pat. No. 4,851,777, issued Jul. 25, 1989 to A. Macovski, disclosing an induction coil arrangement which provides for selective sensing of a response from a localized region;
U.S. Pat. No. 4,857,843, issued Aug. 15, 1989 to A. Macovski, disclosing acquisition of desired spectrographic information in the presence of unknown magnetic fields, using a known signal (from water, for example) as reference signal;
U.S. Pat. No. 4,906,932, issued Mar. 6, 1990 to R. J. Ordidge, disclosing a way of eliminating unwanted signals from spatial or frequency regions by means of radio-frequency noise; and
U.S. Pat. No. 4,942,359, issued Jul. 17, 1990 to K. Sano et al., disclosing a method for in-vivo imaging of blood vessels, involving the elimination of NMR signals due to blood flow.
Typically, with respect to suppression of an unwanted dominant signal, the disclosed methods depend on good homogeneity of the static magnetic field, and they involve the use of additional radio-frequency pulses--which deposit additional power (generating heat) in a sample. In the field of medical diagnostics, however, magnetic field homogeneity may be degraded due to the presence of the patient (whose volume is many liters rather than just cubic centimeters), and radio-frequency heating is potentially harmful.
The invention described in the following provides for suppression of dominant signals (e.g., signals due to water, fat, or both in organic tissue) without reliance on magnetic field homogeneity or on additional radio-frequency pulses.